In a fluidized-bed granulation zone, there are two processes taking place simultaneously: (1) nucleation and (2) growth. Therefore, there is a particle size distribution in the fluidization zone. At a certain operating hot air (or gas) velocity, there is a percentage of small particles ("dust") which is carried out of the bed because the terminal velocity of these particles is lower than the operating velocity of the hot air.
The fine particles or "dust", which are airborn, are separated from the air or gases with a cyclone; most of the fines (over 90%) are separated with the cyclone and the remaining (less than 10%) are captured in the scrubber.
If the fine particles from the cyclone are returned to the fluidization zone as dried solid distinct particles as in Copeland U.S. Pat. No. 3,309,262, some will grow to a sufficient size so that they will remain within the bed. However, a large percentage will simply become airborn as soon as they are introduced to the fluidization zone. Thus, a percentage of fine particles or dust is going around and around; the net result is that larger cyclones are required in order to accommodate this unnecessary circulation of fine particles.
Whereas in the present invention, the quantity of fine particles or "dust" is limited to whatever is generated within the fluidization zone. There is an economy in equipment and a more efficient process.
Another difficulty with the recycling of dried fine particles as described by Copeland is that uniform distribution of the fine particles within the fluidization zone is almost impossible. Distribution is a major problem with large commercial equipment. For example, if the fluidized-bed chamber is 10-15 ft. in diameter, it is impossible to ensure that the recycled dried particles will be distributed uniformly in the fluidization zone. If the dried fine particles are introduced to the fluidization zone with one or two screw feeders on the periphery of the fluidized-bed-chamber as shown by Copeland, most of the particles do not get a chance to grow; they just become airborn as soon as fed to the fluidization zone and end up again in the cyclone. This problem or distribution ("Channeling") is a major one in commercial size equipment and imposes a limit to the maximum diameter of the fluidized-bed chamber.
Whereas by dissolving all the fine particles in the feed solution, the present invention will avoid completely the distribution of "Channeling" problem.
Another problem with the recycling of the fine particles as dried solids is the formation of lumps in the screw feeder or other mechanical devices. The fine particles leaving the cyclone are in the form of dust and as they go from the cyclone to the fluidized-bed, the temperature drops a few degrees. However, because the air is saturated with the solvent (water), condensation takes place. The very small quantity of condensed water and the pressure of the screw feeder cause the "dust" particles to form large lumps. These lumps (up to a few inches in size) can not be fluidized immediately as soon as they are fed to the fluidization zone. The action of the fluidized particles is not sufficient to break them up. Therefore, the lumps fall down on the grid.
Since the grid is usually very hot (500.degree.-600.degree. F.), the material may melt or decompose. The fluidization zone temperature could be only 150.degree.-250.degree. F. If we keep the grid temperature low (below the point of melting or decomposition) then the quantity of air or hot gases must be increased and, in turn, the size of the equipment.
In work with ammonium sulfate which decomposes below 300.degree. F. it has been found that lump formation, as described above, must be avoided under all circumstances in order to have a feasible continuous process.
The prior art has not recognized the above problems and for this reason indicates recycling of most fine particles as dried solids. Whereas, according to the present invention it is found that in order to eliminate the above problems, all the fine particles ("dust") should be dissolved in the feed solution and then fed to the fluidization zone.